Fix getRelationsForEvent under TypeScript strict mode (#9558)

* Fix getRelationsForEvent tsc strictness

* Use shared type for GetRelationsForEvent

* Fix lint

* Add alternative type

* getRelationsForEvent is not required

* Relations are optional

* Reactions are optional

* We expect relations in these tests

* Add more protection if the eventID is not defined

* Allow null too

* Better test typing

* User ID is not necessary unless something is selected

* It's okay to [].includes(null)

* Null is as good as undefined here

* Null or undefined is good here

* We have some expectations for the tests

* The room and user can be undefined too

* Protec

* Reactions are optional

* Try match signatures

* Null or undefined

* More null or undefined

* Protec

* Fix typo (wrong variable)

* Remove optional params

See https://github.com/matrix-org/matrix-react-sdk/pull/9558#discussion_r1017515913

* Fix up last maaaaybe relevant lint

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/structures/TimelinePanel.tsx

@@ -1714,7 +1714,7 @@ class TimelinePanel extends React.Component<IProps, IState> {
 
     private getRelationsForEvent = (
         eventId: string,
-        relationType: RelationType,
+        relationType: RelationType | string,
         eventType: EventType | string,
     ) => this.props.timelineSet.relations?.getChildEventsForEvent(eventId, relationType, eventType);